Literacy seems to make sense to me but I might be missing something in the post. Writing is language and language is communication so at least two sides.
As more people learned to read, they also learned to write, and written communications increases. However, even with modest literacy one can read a long sentence. Or can do that when it is written by a good/​skilled writer. But being able to read does not really lead to writing skills in most cases I suspect.
As more people started communicating via writing (think things like schools and education expansion) the skill level of the average writer likely declined. That probably lead to training next generation writes to write in a more simple sentence structure.
Literacy seems to make sense to me but I might be missing something in the post. Writing is language and language is communication so at least two sides.
As more people learned to read, they also learned to write, and written communications increases. However, even with modest literacy one can read a long sentence. Or can do that when it is written by a good/​skilled writer. But being able to read does not really lead to writing skills in most cases I suspect.
As more people started communicating via writing (think things like schools and education expansion) the skill level of the average writer likely declined. That probably lead to training next generation writes to write in a more simple sentence structure.